While the apparatus of this invention is applicable to a wide range of devices wherein a rotatable shaft extends through a wall, said wall usually being a portion of a closed vessel in which a fluid pressure is created, it is particularly applicable to such devices as pumps, autoclaves, turbines and the like. Accordingly, for purposes of discussion, reference will be made hereinafter to the use of such invention with pumps. Such reference will, however, be understood as being illustrative only and imparting no limitation on the use or applicability of the invention.
Mechanical seals for rotating shafts, such as in or adjacent the stuffing box of a centrifugal pump, normally are grouped into two well-known classes which are commonly termed "inside seals" and "outside seals" depending on whether the sealing faces are inside or outside the stuffing box. Typical "inside" seals are illustrated in U.S. Pat. Nos. 3,765,689, 3,697,088 and 3,665,206; while typical "outside" seals are illustrated in U.S. Pat. Nos. 3,155,393 and 3,198,530.
In devices which handle fluids generally considered dangerous, such as due to the high temperature, high pressure and/or corrosive properties of the fluid, inside seals have been substantially unanimously utilized since, for reasons well understood by those skilled in this art, such seals are considered more satisfactory from such standpoints as lubrication, capacity to withstand high pressures, capability of controlling seal temperature while handling high temperature fluids, and ability to prevent sudden and uncontrolled discharge of a dangerous fluid in the event of seal failure. Inside seals, however, do have a problem when handling high temperature fluids, such as heat transfer fluids above 300.degree. F., which fluids can oxidize and harden upon contact with air. The hardened product leakage accumulates between the shaft and rotary seal part, soon causing immobilization (or more commonly called "hang-up") of the axial movement necessary to allow for wear, and the seal fails by leakage. Normal outside seals have not been considered adaptable to providing compliance or improvement on the above features.
More specifically, particularly when dealing with fluids of high temperature, such as temperatures in excess of approximately 300.degree. F., cooling of the seal becomes a significant problem since the heat of the fluid within the pumping chamber is transmitted to the seal faces, both through the fluid and through the shaft. The overall seal thus becomes relatively hot and this results in damage to the seal assembly, and specifically the packings and gaskets, and the formation of a hard coke-like deposit from normal minute leakage which tends to build up and immobilize the movable sealing element which in turn creates more leakage, soon causing the seal to fail. To control the temperature, inside seals are particularly suitable since the seal faces are located within the stuffing box and a cooling chamber is generally provided in surrounding relationship to the seal, which cooling chamber has a separate liquid coolant circulating therethrough for controlling seal temperature, often by flushing over the rubbing faces through the gland. Seals of the outside type, on the other hand, due to their being positioned outside the stuffing box, are thus not readily adaptable for use with a separate liquid coolant for permitting the seal temperature to be controlled.
Outside seals are more desirable for minimizing the effects of coke-like deposits on the atmospheric side of the seal faces because the deposit is flung outward by centrifugal force, whereas with an inside seal, the centrifugal force tends to trap the leaked product which hardens and causes immobilization or hang-up of the movable element; hence the need to cool the product is more urgent with the inside seal.
In addition, outside seals normally require the use of an elastomeric O-ring at the outer end of the seal for creating a sealed relationship between the rotatable seal ring and the shaft. When fluid temperatures exceed 300.degree. F., conventional elastomeric O-rings deteriorate at a rather rapid rate and thus ultimately result in leakage, which not only creates a potentially dangerous condition, but also requires undesirable shutdown of the equipment and repair of the seal. For this reason, in conjunction with the other disadvantages explained above, seals of the outside type have thus normally been usable only with relatively low temperature fluids. For example, in the refinery industry wherein the fluids being handled normally are at relatively high temperatures, the pumping equipment almost exclusively utilizes inside seals in view of the recognized disadvantages and problems experienced with outside seals.
Thus, while the applicability of outside seals has heretofore been extremely limited, nevertheless such outside seals are well recognized as possessing advantageous features in contrast to inside seals. For example, the outside seal is advantageous from the standpoint of visual inspection, access, maintenance and repair, and is much less prone to hang-up problems created by coking-type fluids such as heat transfer fluids and hydrocarbons. However, in view of the shortcomings mentioned above, outside seals have had only limited applicability.
Accordingly, it is an object of this invention to provide an improved outside seal construction which overcomes many of the above-mentioned shortcomings and, in particular, is highly desirable for use with high temperature fluids, that is fluids having a temperature in excess of approximately 300.degree. F. More specifically, the objects of the present invention are the provision of:
1. An improved outside seal construction, as aforesaid, which can be safely and satisfactorily used with high temperature fluids while at the same time being substantially self-cooling, so as to not require the use of complex external cooling systems.
2. An outside seal construction, as aforesaid, which employs a thermally conductive packing disposed in surrounding relationship to the shaft and located axially between the seal faces and the pumping chamber, which packing is of high radial thermal conductivity but creates a thermally insulated barrier in the axial direction thereof, whereby heat is readily transmitted radially from the shaft and fluid through the packing into the stuffing box and thence into the environment.
3. An outside seal construction, as aforesaid, wherein the packing preferably employs a plurality of packing rings disposed in axially aligned relationship along the shaft, with thermally insulative washers being disposed between axially adjacent packing rings, whereby the shaft and fluid heat is readily transmitted radially outwardly of the stuffing box.
4. An outside seal construction, as aforesaid, which is also desirable for use with high pressure fluids while providing for safe confinement of the fluid in the event of seal failure.
5. An outside seal construction, as aforesaid, wherein the packing is axially elongated and closely surrounds the shaft to define an axially elongated annular clearance space of narrow radial width therebetween, whereby the packing functions as a restrictive bushing and hence prevents the rapid escape of large quantities of fluid therepast in the event of a seal failure.
6. An outside seal construction, as aforesaid, which permits a rotatable fan structure to be incorporated therewith to facilitate and assist in directing the flow of air across the exterior of the seal and stuffing box to facilitate self-cooling of the seal.
7. An outside seal construction, as aforesaid, which utilizes a pair of opposed relatively rotatable face rings for defining a sealed engagement therebetween, which face rings are of substantially the same exterior diameter so that any fluid which tends to escape by flowing radially outwardly between the seal faces is thus more easily discharged from the seal, such as by being thrown off due to centrifugal force, whereby the fluid does not harden directly adjacent the outer edges of the seal faces and thereby cause hang up of the seal.
8. An outside seal construction, as aforesaid, which possesses all of the conventional advantages normally possessed by an outside seal, while at the same time also possessing many of the operational advantages previously possessed only by an inside seal.
Other objects and purposes of the invention will be apparent to persons familiar with seals of this type upon reading the following specification and inspecting the accompanying drawings.